15 Sep 2011: Analysis

The Crucial Role of Predators: A New Perspective on Ecology

Scientists have recently begun to understand the vital role played by top predators in ecosystems and the profound impacts that occur when those predators are wiped out. Now, researchers are citing new evidence that shows the importance of lions, wolves, sharks, and other creatures at the top of the food chain.

by caroline fraser

Found in the North Palace at Ninevah, stone panels depicting the Royal Lion Hunt of the last Assyrian king, Ashurbanipal, are as violent as any video game: A female lion flies upside down, arrows protruding from her back and belly. Beneath her, a male rears back, arrows piercing his nasal passages while another male drags his hindquarters behind him. From the king’s chariot, attendants drive spears through the chest of another.

The panels are two-and-a-half thousand years old, and the story they tell is nearly over. In Africa, the lion’s numbers have declined sharply in the past decade, to as low as 23,000. The tiger is near extinction. Earlier this year, a mountain lion walked 1,800 miles from the Black Hills of South Dakota to the East Coast — one of the world’s longest recorded journey by a land mammal — only to be killed by a sport utility vehicle near Milford, Connecticut, 50 miles from New York City.

Just as the world’s lions, tigers, and bears are disappearing worldwide, a scientific consensus is emerging that they are critical to ecosystem function, exerting control over smaller predators, prey, and the plant world. Studies

Experts on predation have become increasingly convinced that ecosystems are ruled from the top.

of predation — a so-called “top-down” force in nature — have always run a weak second to ecology’s traditional focus, which holds that the foundation of life springs from bottom-up processes enabled by plants capturing energy from the sun. While no one disputes the importance of photosynthesis and nutrient cycling, experts on predation have become increasingly convinced that ecosystems are ruled from the top.

Beginning with aquatic experiments, they have amassed considerable evidence of damage done to food chains by predator removal and have extended such studies to land: Predation may be as consequential, if not more so, than bottom-up forces. With a comprehensive new book (Trophic Cascades) and a major Science review published this summer, these specialists present the case that our persecution of predators menaces the marine and terrestrial ecosystems that produce food, hold human and zoonotic diseases in abeyance, and stabilize climate.

Using such terms as “deep anxiety” and “grave concern” to signal their alarm, the authors contend that the loss of large animals, and apex predators in particular, constitutes humanity’s “most pervasive influence” on the environment. It amounts, they argue, to a “global decapitation” of the systems that support life on Earth.

These are hardly new ideas: Both publications catalogue decades of work examining the power of predators. Charles Elton, an Oxford ecologist, first conceptualized food webs in the 1920s, speculating that wolf removal would unleash hordes of deer, a notion that weighed on Aldo Leopold’s mind as he compared the consequences of wolf-extirpation in German forests to still-thriving, intact systems in Mexico’s Sierra Madre Mountains.

U.S. Fish & Wildlife

The return of wolves to Yellowstone proved that damage to a terrestrial food web could be restored.

These insights gave rise to the 1960s “green world” hypothesis, which held that plants prevail because predators hold herbivores in check. Profound food chain effects — caused by adding or removing top species — are now known as “trophic cascades.” In a classic 1966 experiment, biologist Robert Paine removed the purple seastar, Pisaster ochraceus — a voracious mussel-feeder — from an area of coastline in Washington state. Their predator gone, mussels sprouted like corn in Kansas, crowding out algae, chitons, and limpets, replacing biodiversity with monoculture.

Corroborating evidence multiplied. Less than a decade after Pisaster, marine ecologists James Estes and John Palmisano reached the astonishing and widely reported conclusion that hunting of sea otters had caused the collapse of kelp forests around the Aleutian Islands. While the cat was away, the prey (sea urchins) stripped the larder bare. When otters returned, they regulated urchins, allowing “luxuriant” regrowth of biodiverse kelp communities. Around islands farther out to sea, where the mammals had not reestablished themselves, “urchin barrens” remained.

The Science review this summer and other recent research have highlighted the cost of cascades in other marine systems. Extirpation of great sharks along the eastern seaboard caused an irruption of rays and the collapse of a century-old scallop fishery, a glimpse of the future as shark populations crash worldwide. Overfishing of cod, a top predator of lobster and sea urchins, upended the coastal North Atlantic, producing hyper-abundant lobster and a market glut in the Gulf of Maine, as well as an urchin boom-and-bust cycle off Nova Scotia, where urchins have been periodically wiped out by disease.

Yet, as data from aquatic systems proliferated, skeptics suggested that top-down forces might be “all wet” — limited to marine or freshwater systems, with a dearth of evidence for cascades in terrestrial systems.

Where was that evidence? Designing experiments to reveal cascades on land, across large-scales and over long time periods, seemed nearly impossible. So many ecosystems had already been irreparably altered that predator-related effects — including damage done to food chains, so-called “trophic downgrading” — could not be measured with certainty. Long-term trials teasing out wide-ranging interactions among predators and other species promised to be unwieldy and expensive.

Nonetheless, startling revelations continued to crop up. In a Venezuelan valley flooded by construction of a dam in the 1980s, Duke University ecologist John Terborgh and his students documented the strange

‘We have to pay attention to the well-being of predators if we want a healthy ecosystem,’ says one scientist.

perturbations that afflicted the “islands” of Lago Guri. Top predators — jaguar, mountain lion, harpy eagle — fled rising waters. Multiplying out of control, howler monkeys went mad as their numbers soared and the plants they ate increased toxins in self-defense. Some islands were cloaked in thorns as leaf-cutter ants — undeterred by armadillos or other predators — starved the soil of nutrients by carrying every leaf down to their lairs.

In 1995, the terrestrial camp landed an extraordinary boon as Yellowstone National Park gave William Ripple, director of Oregon State University’s Trophic Cascades Program, the chance to study top-down forcing in action. Ripple watched in amazement as the wolf’s return to Yellowstone — an ecosystem where elk had had the browse of the place for 75 years — gave willow and other trees the chance to take hold along stream banks, cooling water temperatures for trout and encouraging the return of beaver, whose ponds host long-absent amphibians and songbirds. Yellowstone proved that damage to a terrestrial food web could be reversed and an ecosystem restored with the return of a single species. It is a sobering lesson for the eastern U.S., where the explosion of white-tailed deer has eradicated hemlock, a keystone species in once-biodiverse hardwood forests.

Yet despite such developments, researchers of trophic cascades have despaired of securing the money and means to examine predator removal in large-scale, long-term trials on land. Some have dealt with constrictions by adopting a more manageable, meadow-sized scale. In a three-year experiment, ecologist Oswald Schmitz of the Yale School of Forestry & Environmental Studies found that even the tiniest of predators (spiders) exercise a more significant top-down influence on plants than bottom-up factors. The type of predation — active versus ambush hunting — also appears to be consequential, affecting the composition of plant communities and nitrogen levels. Spiders that hunt actively reduce grasshopper density, allowing grass and goldenrod to dominate other plants and increasing available nitrogen. Ambush hunting has an opposite effect, forcing grasshoppers, which would rather feed on grass, to shelter in goldenrod, yielding a more diverse plant community and less nitrogen. Taken together, Schmitz says, “it’s the richness of the functional role of predators that becomes important to conserve.”

Estes and Terborgh, editors of Trophic Cascades, question whether spiders and grasshoppers will “convince anyone that orcas, great white sharks,

‘The idea that plants are affected by the things that eat them has not been widely appreciated,’ says one expert.

wolves, tigers, and jaguars are important.” But Schmitz, who grew up north of Toronto where wolf-hunting was a way of life, thinks the process is underway: “Piece by piece, it’s taken 20 years to accumulate the evidence, and the culmination is in that Science paper — that the world is driven by predators as well as nutrients. We have to pay attention to their health and well-being if we want a healthy ecosystem. Simply eliminating them because we want more prey or because we don’t think they’re important is very misguided.”

Indeed, the Science review presses the trophic case into new territory, extending predation’s impact to human health. A reduction in lion and leopard populations in Ghana has led to an explosion of olive baboons. The release of such “mesopredators” — mid-sized carnivores such as cats or raccoons that run rampant without control — has wreaked havoc around local villages, where baboons attack livestock, damage crops, and spread intestinal parasites to the human population.

In the Science paper, the authors call for “a paradigm shift in ecology.” Scientists and land-managers, they argue, must adopt top-down forcing as a given “if there is to be any real hope of understanding and managing the workings of nature.”

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In Trophic Cascades, Terborgh and Estes go farther, criticizing national science agencies for failing to fund research on predator removal in terrestrial systems, accusing them of clinging to old views and “retarding progress” while ecosystems are undermined. “The idea that plants are affected by the things that eat them,” Estes says dryly, “has not been widely appreciated.”

But Alan James Tessier, program director of the National Science Foundation’s Environmental Biology Division, disagrees, asserting that the agency has funded much research into top-down processes. “It’s ridiculous to talk only about top-down or bottom-up control,” said Tessier. “Both are happening all the time.”

In science, new ideas are rightly met with skepticism, if not denials and dismissals. But as the consequences of predator loss become increasingly measurable and predictable, they implicitly call for a reassessment of our ancient foes. Estes is as reluctant as any scientist to weigh in on the wolf wars, but his frustration is clear. “That’s not the way we should be behaving as a species,” he says.

COMMENTS

The central ecological problem is that humans have no predators and no money or scientific program will help to solve this issue.

It is not by reintroducing a bunch of wolves here and there that we will reduce the apocalyptic artificialization of our world.

May be this is why there are fiercer and fiercer wars. In the end, Homo homini Lupus.

Posted by
kervennic
on 15 Sep 2011

This is an excellent article and high time that the subject was brought to national attention - Thank you!

The Wolf's Tooth by Christina Eisenberg also has an excellent take on the trophic cascade issue. One small item in the article is, I believe, misleading. A keystone species, strictly speaking and as descirbed by Paine, cannot be anything less than a top predator, not a hemlock species as described in the text.

Posted by
Steve Carballeira
on 15 Sep 2011

Estes et al.'s seminal article about trophic downgrading invites us to reconsider the type of world we are creating, in which we are severing long-standing evolutionary relationships between predators and their prey. These truncated relationships create implications that reverberate through whole ecosystems. In whatever ecosystem you consider, mending the web of life can be as simple as restoring and creating sustainable populations of keystone predators.

Steve Carballeira is correct. In my book "The Wolf's Tooth," also published by Island Press, I use Paine's definition of a keystone species, which is a top carnivore that preferenctially preys on a particular prey species. According to Paine, keystone effects further necessitate that the prey species be a dominant species in its trophic class. The term "keystone species" has since been applied to a variety of things, including prairie dogs, bison, and "keystone" ecosystems. This is not how Paine originally intended this term to be used.

Posted by
Cristina Eisenberg
on 15 Sep 2011

Should our relationship with the rest of the living world be based on an ideology of conservation biology - or compassion? Ethically, I can see no more case for trying to preserve nonhuman predators than to preserve human predators: they both cause horrific suffering. Control of herbivore populations in our future wildlife parks can best be maintained by immunocontraception and other forms of fertility control - rather than by famine, disease and the cruelties of predation as now. Humans already massively interfere with Nature through habitat destruction, captive breeding programs for big cats, "rewilding" etc. Instead let's phase out being eaten alive, disemboweled and asphyxiated in favour of compassionate stewardship.

Posted by
David Pearce
on 15 Sep 2011

I realize that there have been debates over the use (and sometimes overuse) of the term "keystone species." Readers may be interested in the discussion of hemlock as a keystone in Chapter 9 of Estes and Terborgh's "Trophic Cascades": "Large Predators, Deer, and Trophic Cascades in Boreal and Temperate Ecosystems," by William J. Ripple, Thomas P. Rooney, and Robert L. Beschta. In that chapter, the authors refer to hemlock as "a keystone species that contributes to regional biodiversity," citing a paper by Ellison et al. that appeared in "Frontiers in Ecology and the Environment" in 2005.

Posted by
Caroline Fraser
on 15 Sep 2011

"Extirpation of great sharks along the eastern seaboard caused an irruption of rays and the collapse of a century-old scallop fishery." This statement is absolutely incorrect. Scallops crashed due to loss of HABITAT (seagrasses), not due to cownose rays. This is well-documented in the literature. As for the irruption of rays along the eastern seaboard, this is impossible to prove or disprove since there has never been a stock assessment of this species.

Posted by
Dr. Charles Cotton
on 15 Sep 2011

Yes, species need to be recognized for their key roles. But the boundaries of the definition are a little shaky. Beavers ought to be considered a keystone species for the way they alter the landscape and attract whole new assemblages of plants and animals. Of course, they used to have prevalent predators, and now largely lacking them, they continue to make the landscape more interesting. Geology could as well be considered a keystone, no pun intended.

Paine would not approve.

Posted by
Erik Hoffner
on 15 Sep 2011

The greatest problem with us as a species is that we have not evolved enough consideration to keep pace with our other abilities, the result being that we do things simply to prove that we can; we never consider the possible aftermath, or how other creatures may respond in time. The ecosystem has had to suffer for this, and for the fact that we never long for very long-term aftereffects, such as with DDT. Poaching is a more direct assault, and should, likewise, be dealt with directly.

The greatest shame of alls that we never think before we act, not of other people, and rarely of other species. Each of us can at least start there.

Posted by
Alex
on 16 Sep 2011

Cristina, do you really think that wildlife exploding in numbers and subsequently starving to death or wasting away from disease is more ethical than predators killing them? Furthermore, what of all the creatures lost through the subsequent reduction in biodiversity?

I'm not a philosopher, but your argument is nonsensical.

Posted by
Tor Bertin
on 16 Sep 2011

Caroline, I would like to thank you for including, if albeit brief, Aldo Leopold in this article. On many occasions this great ecologist's thoughts and findings are overlooked.

Posted by
Joshua
on 16 Sep 2011

Some truths are "self-evident." It's taken science this long to figure out the vital contribution of predators to the survival of all species?

Posted by
ron tansky
on 17 Sep 2011

The abstract might suggest that "scientists have recently," but the thesis is well known to many, for ages, from those living near the land to those wonderful drawings from the 1960s depicting "apex predators." All creatures in the web of life play a role. While the most important may in fact be earthworms, the ecological impact of the loss of top predators was often ignored due to lazy thinking and social convention. It's nice to see science wake up from both - but it's not "new."

Posted by
Paul Fitzpatrick
on 18 Sep 2011

Brilliant article! My clear and present concern is the Yellowstone wolves. Where are the scientists who re-introduced this species? The wolves of Yellowstone are at HIGH RISK of being trapped, gassed and shot on site in the coming months. It is unethical to re-introduce an apex predetor (or any species) just to shoot and kill it later. One of my proposed solutions is Humane TNR (Trap, Neuter and Release). The barbaric hunting and killing practices of the past should be left just there, in the past. We are more intelligent than that! I hope the scientists who re-introduced the WOLF into North America will take FULL responsibility for their lives and the lives of their offspring.

Posted by
MA Moore
on 18 Sep 2011

Because of a link through the Society for Ecological Restoration's weekly news summary, additional folks will see this article. It is a very important ecological topic that our political leaders and policy makers should also be exposed to.

Posted by
Ted Harris
on 21 Sep 2011

Brilliant article! But I agree with Paul Fitzpatrick; this is not new!

I was under the impression that a keystone species is supposed to represent a species that has a disproportionate effect of the functioning of an ecosystem relative to its abundance. Under this definition, any species – from worm to whale and any plant – can be a keystone species. This seems logical to me as a keystone in an arch is the stone that individually has a huge impact on stability of the arch – which I thought is the context in which this term was coined.

Posted by
Ryan Miller
on 10 Oct 2011

It's nice reading them all however what interest me to read know understand at current and future times would be on ecosystem studies that affects now intolerable atmospheric carbon dioxide emission how to limit them to address global warming. We believe latter control is the one as more essential to all living creature's preservation.

Posted by
Samuel C. Hechanova, Jr., M.S.
on 15 Oct 2011

Thank you Caroline Fraser for a simply brilliant, informative and extremely educational article, which we would all do well to read, re-read and share with our members of Congress who seem hell-bent to turn our nation into a national stockyard.

I note with interest MA Moore's comment regarding the responsibility of the scientists who reintroduced the gray wolf to Yellowstone. On March 30, 2011 1,293 Scientists with Expertise in Biological Systems wrote to the entire Senate “to urge them to vote against any legislation that would undercut the best available science as the basis for adding or removing any particular species from the protection of the Endangered Species Act. Allowing Congress to remove or add protections for particular species would set a dangerous precedent, as the fate of every species on the endangered species list (or any candidate for that list) would then be subject to political interference.”

It is the fault of President Obama and 81 Senators who chose to ignore that request, allowing rider Section 1713 to remain in a must pass spending bill.

Posted by
Rhonda Lanier
on 28 Oct 2011

Thank you for the extremely important information that has assisted me in doing my university assignments in Environmental Resource Conservation. May god bless you

Posted by
ESTHER CHANGE
on 01 Nov 2011

We should, everywhere, protect every species, EVERY animal, EVERY mammal, small or large, should be put on the extinction list. Over fishing, trophy hunting, pollution, land clearing and population boom EVERY PLACE on earth. IT is devastating! We have known about this information for a long time and have not paid attention.

Posted by
Kathy lawrence
on 18 Feb 2012

In response to the first comment: "The central ecological problem is that humans have no predators and no money or scientific program will help to solve this issue."

So essentially we as humans need a significant natural predator to control our population... would this possibly bring us back into some kind of intimate relationship with the earth and our ecosystem again?

Unfortunately there are no more predators and unless there suddenly appeared large breeding populations of velociraptors firmly entrenched throughout the globe it doesn't seem as there
could ever be a natural predator to man. The responsibility lies within ourselves as a knowledgeable and sentient species (with the technology and power to wield it) to manage our
environment to the best of our abilities. The consequences are alarmingly obvious as to the impacts our actions have on the world around us.

Education seems the only solution and change will probably only come from the next generation if they are made aware of the issues....

...although the Earth has had its reset button hit before as evidenced in the fossil record.... kinda lame if it was our fault and we knowingly did it.

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